Solar Cell And Production Thereof

ABSTRACT

A solar cell is configured to include: a substrate ( 21 ); a conductive film ( 22 ) formed on the substrate ( 21 ); a compound semiconductor layer ( 23 ) formed on the conductive film ( 22 ), including a p-type semiconductor crystal containing an element of Group Ib, an element of Group IIIb, and an element of Group VIb; a n-type window layer ( 24 ) formed on the compound semiconductor layer ( 23 ), having apertures ( 29 ); and a n-type transparent conductive film formed on the n-type window layer ( 24 ) and on portions of the compound semiconductor layer ( 23 ) at the apertures of the n-type window layer ( 24 ). The compound semiconductor layer ( 23 ) includes high-resistance parts ( 23 B), in portions of the compound semiconductor layer ( 23 ) in the vicinity of a surface thereof on a side opposite to the conductive film ( 22 ), and the high-resistance parts ( 23 B) contain a n-type impurity doped in the p-type semiconductor crystal. The high-resistance parts ( 23 B) are located under the apertures ( 29 ) of the n-type window layer ( 24 ), respectively.

TECHNICAL FIELD

The present invention relates to a solar cell. The present inventionparticularly relates to a substrate-type solar cell.

BACKGROUND ART

Conventionally, a solar cell of the substrate type has been known, whichincludes a substrate, a lower electrode formed on the substrate, alight-absorption layer formed on a conductive film, a window layerformed on the light-absorption layer, and an upper electrode formed onthe window layer. Further, a solar cell of the substrate type thatfurther includes a buffer layer formed between the light-absorptionlayer and the window layer has been known.

Regarding the conventional solar cell of the substrate type, morespecifically, a configuration has been proposed that includes a glasssubstrate containing an alkali metal such as Na, a metal film (lowerelectrode) such as a MO film formed on the glass substrate by sputteringor the like, a chalcopyrite-structured compound semiconductor layer(light-absorption layer) having a p-type conductivity such as a p-typeCu(In,Ga)Se₂ layer, which is formed on the metal film by multisourcevapor deposition or the like, a CdS layer (window layer) formed on thecompound semiconductor layer by a solution method, and a n-typetransparent conductive film (upper electrode) such as a ZnO:Al film(see, for instance, JP10(1998)-74967A). To produce a solar cell with ahigh energy conversion efficiency, in the conventional configuration,the p-type Cu(In,Ga)Se₂ layer functioning as the light-absorption layerwas formed by applying a method in which a p-type Cu(In,Ga)Se₂ crystalwas grown slowly over a long time. This is because the slow crystalgrowth allows not only for the reduction of crystal defects in thep-type Cu(In,Ga)Se₂ layer, but also the enhancement of the flatness ofthe surface thereof even though the layer is polycrystalline. Further,by forming a CdS layer on the p-type Cu(In,Ga)Se₂ layer with the flatsurface, the CdS layer can be formed with excellent coverage.

Further, another example of the conventional substrate-type solar cellhas been proposed that includes a glass substrate, a metal film (lowerelectrode) such as a MO film formed on the glass substrate by sputteringor the like, a chalcopyrite-structured compound semiconductor layer(light-absorption layer) having a p-type conductivity such as a p-typeCu(In,Ga)Se₂ layer, which is formed on the metal film by a selenidationmethod, a buffer layer such as ZnO film formed on the compoundsemiconductor, a window layer such as a ZnO:Al film, and an upperelectrode (see, for example, JP10(1998)-135498A). In the selenidationmethod, the p-type Cu(In,Ga)Se₂ layer may be formed by forming aCuGa/In/Se precursor (a stacked film composed of a CuGa film, an Infilm, and a Se film) and thereafter causing solid phase diffusion of theCuGa/In/Se precursor by heating, or alternatively, the p-typeCu(In,Ga)Se₂ layer may be formed by forming a CuGa/In precursor andthereafter subjecting the same to heat treatment in H₂Se gas.

Further, still another example of the conventional substrate-type solarcell has been known that includes, as a buffer layer, a Zn(O,H) film, aZn(O,S,OH) film, or the like formed by the solution method (see, forinstance, Tokio Nakada et al., “Thin Solid Film” 431-432 (2003)242-248).

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

In the formation of a p-type Cu(In,Ga)Se₂ layer by multisource vapordeposition, if the layer is grown slowly as conventionally, even theformation of the p-type Cu(In,Ga)Se₂ layer with only a thickness of 2 μmtakes approximately one hour, which results in extremely low massproductivity. On the other hand, if the p-type Cu(In,Ga)Se₂ layer with athickness of 2 μm is grown rapidly over about several minutes, a n valueas a diode index exceeds 2, thereby causing the obtained solar cell tohave an energy conversion efficiency below 10%. This is because in thecase where the p-type Cu(In,Ga)Se₂ layer is formed rapidly, thecrystallinity of the p-type Cu(In,Ga)Se₂ layer deteriorates. Further,this also is because the p-type Cu(In,Ga)Se₂ layer has an uneven surfacewithout flatness on its face. Still further, this is because if then-type CdS layer is formed on the p-type Cu(In,Ga)Se₂ layer having theuneven surface on its face, the n-type CdS layer provides onlyinsufficient coverage, which leads to a decrease in a shunt resistancein an equivalent circuit. This decrease of the shunt resistance stemsfrom a phenomenon in which since a high-concentration n-type ITO film isformed on the n-type CdS layer providing insufficient coverage, a partof the n-type ITO film is in direct contact with the p-type Cu(In,Ga)Se₂layer, not via the n-type CdS layer. This phenomenon causing thedecrease of the shunt resistance occurs not only in the case of thep-type Cu(In,Ga)Se₂ layer, but also in the cases of the otherlight-absorption layers formed rapidly by multisource vapor deposition.

Further, also in the formation of a light-absorption layer such as thep-type Cu(In,Ga)Se₂ layer by the selenidation method, if the CuGa/In/seprecursor layer or the CuGa/In precursor layer is grown rapidly overabout several minutes, the layer has an uneven surface on its face,which causes the light-absorption layer such as the p-type Cu(In,Ga)Se₂layer, which is formed finally, to have an uneven surface withoutflatness on its face. Therefore, also in the case where a n-type bufferlayer or a n-type window layer is formed on the light-absorption layerformed by the selenidation method, the coverage of the n-type bufferlayer or the n-type window layer becomes insufficient, which decreases ashunt resistance in an equivalent circuit. The decrease in the shuntresistance stems from a phenomenon in which, because a n-typetransparent conductive film is formed on the n-type buffer layer or then-type window layer providing the insufficient coverage, a part of then-type transparent conductive film is in direct contact, not via thelight-absorption layer without both of the n-type buffer layer or then-type window layer.

To cope with this problem, in a solar cell that has a compoundsemiconductor layer (light-absorption layer) that is formed rapidlythereby having an uneven surface and a n-type window layer that isformed on the compound semiconductor layer thereby having insufficientcoverage, the present invention improves solar cell properties such asan energy conversion efficiency by increasing a shunt resistance in anequivalent circuit of the solar cell, in other words, by decreasing aleak current of the solar cell.

Means for Solving Problem

In order to solve the above-described problems, a solar cell of thepresent invention includes: a substrate; a conductive film formed on thesubstrate; a compound semiconductor layer formed on the conductive film,including a p-type semiconductor crystal containing an element of GroupIb, an element of Group IIIb, and an element of Group VIb; a n-typewindow layer formed on the compound semiconductor layer, having anaperture; and a n-type transparent conductive film formed on the n-typewindow layer and on a portion of the compound semiconductor layer at theaperture of the n-type window layer. The solar cell is characterized inthat the compound semiconductor layer includes a high-resistance part ina portion of the compound semiconductor layer in the vicinity of asurface thereof on a side opposite to the conductive film, thehigh-resistance part containing a n-type impurity doped in the p-typesemiconductor crystal, and that the high-resistance part is locatedunder the aperture of the n-type window layer. Herein the Groups ofelements are referred to according to the short-form periodic tablerecommended by the International Union of Pure and Applied Chemistry(IUPAC). It should be noted that “Group Ib”, “Group IIIb”, and “GroupVIb” refer to “Group 11”, “Group 13”, and “Group 16” according to thelong-form periodic table recommended by IUPAC. The n-type impurityrefers to an element that functions as a donor when being doped in thep-type semiconductor crystal.

In order to solve the above-described problems, a solar cell producingmethod of the present invention includes the steps of forming aconductive film on a substrate; growing a p-type semiconductor crystalon the conductive film, the p-type semiconductor crystal containing anelement of Group Ib, an element of Group IIIb, and an element of GroupVIb; forming a n-type window layer on the p-type semiconductor crystal,the n-type window layer having an aperture; and forming a n-typetransparent conductive film on the n-type window layer and on a portionof the p-type semiconductor crystal at the aperture of the n-type windowlayer, and the solar cell producing method is characterized by furtherincluding the step of doping an n-type impurity in the p-typesemiconductor crystal, in the vicinity of a surface of the p-typesemiconductor crystal under the aperture of the n-type window layer, thedoping step being carried out between the step of forming the n-typewindow layer and the step of forming the n-type transparent conductivefilm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram showing an equivalent circuit of a solarcell according to Embodiment 1.

FIG. 2 is a schematic sectional view showing an example of aconfiguration of a solar cell according to Embodiment 1.

FIGS. 3A to 3D are schematic cross-sectional views showing steps of anexample of a first method for producing the solar cell according toEmbodiment 1.

FIGS. 4A to 4D are schematic cross-sectional views showing steps of anexample of a second method for producing a solar cell according toEmbodiment 2.

FIGS. 5A to 5C are schematic cross-sectional views showing steps of anexample of a third method for producing a solar cell according toEmbodiment 3.

FIG. 6 is a schematic cross-sectional view showing an example of aconfiguration of a solar cell according to Embodiment 4.

DESCRIPTION OF THE INVENTION

As described above, the solar cell of the present invention includes asubstrate, a conductive film, a compound semiconductor layer having ahigh resistance part, a n-type window layer, and a n-type transparentconductive film. It should be noted that a surface of the compoundsemiconductor layer on the n-type window layer side is an unevensurface. The n-type window layer has an aperture (hereinafter alsoreferred to as a “pinhole”), and the high-resistance part is located ina part of a p-type semiconductor crystal in the vicinity of a surfacethereof below the aperture of the n-type window layer.

In the compound semiconductor layer, the high-resistance part is a partformed by doping with a n-type impurity in a part of the p-typesemiconductor crystal in the vicinity of the surface thereof. Here, “thevicinity of the surface” refers to a region with a depth of not morethan 500 nm from a surface of the compound semiconductor layer. Theresistance of the high-resistance part is higher than a resistance of apart other than the high-resistance part in the p-type semiconductorcrystal, the part being not doped with the n-type impurity (hereinafterthis part also is referred to as “low-resistance part”). This is becausethe n-type impurity doped in the p-type semiconductor crystal functionsas a donor, which results in an increase in the donor concentration inthe high-resistance part as compared with the low-resistance part and adecrease in a carrier concentration that is determined by an acceptorconcentration and a donor concentration in the p-type semiconductorcrystal.

Here, the equivalent circuit of the solar cell is described withreference to FIG. 1. FIG. 1 is a circuit diagram showing an equivalentcircuit of the solar cell of the present invention. It should be notedthat an equivalent circuit of a conventional solar cell also is shownwith the same configuration as that of the circuit diagram shown inFIG. 1. The equivalent circuit of the solar cell of the presentinvention includes, as shown in FIG. 1, a constant-current source 4(short circuit current J_(sc)), a diode 3 formed with a p-n junctionconnected in parallel with the constant-current source 4, a shuntresistor 1 (resistance R_(sh)) connected in parallel with the diode 3,and a series resistor 2 (resistance R_(s)) connected in series with thediode 3. To obtain a solar cell with excellent properties, it ispreferable that the resistance R_(sh) of the shunt resistor 1 is large,while the resistance R_(s) of the series resistor 2 is small. A decreasein the resistance R_(sh) of the shunt resistor 1 is caused by leakcurrent at the pn junction, leak current due to a crystal defect ordeposition of an impurity in the vicinity of the pn junction, or thelike in the solar cell. An increase in the resistance R_(s) of theseries resistor 2 is caused by increases in resistances, increases inOhmic contact resistances, increases in wire resistances, etc. in therespective layers composing the solar cell. It should be noted thatgenerally the shunt resistor 1 desirably has a resistance of not lessthan 2 kΩ·cm².

With the solar cell of the present invention, an area in which thelow-resistance part and the n-type transparent conductive film are indirect contact with each other can be reduced even if the coverage ofthe n-type window layer is insufficient, and therefore, it is possibleto make the shunt resistor 1 have a high resistance. Accordingly, theproperties of the solar cell such as the energy conversion efficiencycan be improved. Further, by controlling the size of the high-resistancepart and the concentration of the n-type impurity in the high-resistancepart, it is possible to make the shunt resistor 1 have a resistance ofnot less than 2 kΩ·cm².

In the solar cell of the present invention, it is preferable that theresistance of the high-resistance part is higher than a resistance ofthe n-type window layer. In this case, a resistance in a portion inwhich the n-type transparent conductive film and the low-resistance partare in contact with each other with the high-resistance part beinginterposed therebetween can be set higher than a resistance in a portionin which the foregoing two are in contact with each other with then-type window layer being interposed therebetween, and therefore, theleak current at the portion where the n-type transparent conductive filmand the compound semiconductor layer are in direct contact with eachother can be decreased further. In other words, the shunt resistor 1 canbe made to have a higher resistance.

In the solar cell of the present invention, a configuration can beachieved in which the compound semiconductor layer has a recessedsurface on its face on the side opposite to the conductive film, and thehigh-resistance part is formed in the vicinity of the recessed surface.Usually, an aperture in the n-type window layer of insufficient coverageis formed on a recessed portion of the compound semiconductor layerhaving an uneven surface. Therefore, the shunt resistor 1 can be made tohave a high resistance more efficiently by forming the high-resistancepart in the vicinity of the recessed portion. The high-resistance partmay be formed in a portion of the vicinity of the recessed surface, oralternatively may be formed in a larger region incorporating thevicinity of the recessed surface. It should be noted that in the casewhere the uneven surface has many recesses and projections, this becomesa factor for forming many pinholes, as well as provides an effect ofenhancing the energy conversion efficiency since this configurationcauses sunlight incident thereon to be diffused.

In the solar cell of the present invention, the n-type transparentconductive film preferably is connected with a part of the compoundsemiconductor layer (low-resistance part) other than the high-resistancepart only via at least either one of the n-type window layer and thehigh-resistance part. This configuration causes the low-resistance partnot to be in direct contact with the n-type transparent conductive film,thereby allowing the shunt resistor 1 to have a high resistance in anextremely excellent state.

The solar cell of the present invention further may be configured sothat the high-resistance part contains, as the n-type impurity, at leastone element selected from the group consisting of the elements of GroupIIa and the elements of Group IIb. Here, “Group IIa” and “Group IIb”refer to “Group 2” and “Group 12” according to the long-form periodictable recommended by IUPAC. Examples of the configuration of thehigh-resistance part includes, specifically: a configuration in whichone element of Group IIa is contained therein; a configuration in whicha plurality of elements of Group IIa are contained; a configuration inwhich one element of Group IIb is contained; a configuration in which aplurality of elements of Group IIb is contained; and a configuration inwhich at least one element of Group IIa and at least one element ofGroup IIb are contained. With such a configuration, the element(s) ofGroup IIa and/or Group IIb function as a donor doped in the p-typesemiconductor crystal. Further, since the element(s) of Group IIa and/orGroup IIb easily is captured in holes of the p-type semiconductorcrystal that function as acceptors, the foregoing element(s) allows theacceptor concentration to decrease and cause the donor concentration toincrease. Therefore, the foregoing configuration allows the donorconcentration with respect to the acceptor concentration in thehigh-resistance part to increase efficiently. It should be noted thatthe high-resistance part is never caused to exhibit a low-resistancen-type conductivity by an increase in a doped amount of the element(s)of Group IIa and/or Group IIb that tends to cause an extremelyhigh-resistance n-type conductivity to be exhibited. In the solar cellof the present invention, to cause the element(s) to be captured inholes of the element of Group Ib or the like so as to function as adonor excellently, the n-type impurity in the high-resistance partpreferably is Zn, Mg, or Ca.

As the p-type semiconductor crystal in the compound semiconductor layer,a chalcopyrite-structured compound semiconductor crystal containing Cuas the element of Group Ib, at least one element selected from the groupconsisting of Ga and In as the element of Group IIIb, and at least oneelement selected from the group consisting of S and Se as the element ofGroup VIb is preferred. With this configuration, a solar cell isachieved that exhibits a high energy-conversion efficiency and thatsubstantially does not deteriorate over time due to irradiation withlight. More specifically, in the solar cell of the present invention,for instance, the p-type semiconductor crystal of the compoundsemiconductor layer preferably is a chalcopyrite-structured CuInSe₂crystal, a chalcopyrite-structured Cu(Ga,In)Se₂ crystal, or achalcopyrite-structured CuIn(S,Se)₂ crystal. It should be noted that inthe low-resistance part and the high-resistance part, other elements maybe contained as required, as long as they do not harm the effects of thepresent invention.

In the solar cell of the present invention, the n-type window layerpreferably is a ZnO film or a ZnMgO film. It should be noted that then-type window layer in the solar cell of the present invention may havethe same configuration as that of a n-type window layer of any knownsolar cell.

The solar cell of the present invention may be configured so as toinclude further a n-type buffer layer formed between the compoundsemiconductor layer and the n-type window layer, the n-type buffer layerhaving an aperture that is connected with the aperture of the n-typewindow layer. With this configuration, even in the case where thecompound semiconductor layer and the n-type transparent conductive filmare brought into contact with each other via the connected aperturegoing through the n-type window layer and the n-type buffer layer, thearea in which the low-resistance part and the n-type transparentconductive film are in direct contact with each other can be reduced.Accordingly, the shunt resistor 1 is allowed to have a high resistance.In the solar cell of the present invention, the buffer layer preferablyis a Zn(O,OH) film or a Zn(O,S,OH) film. It should be noted that then-type buffer layer in the solar cell of the present invention may havethe same configuration as that of a n-type buffer layer of any knownsolar cell.

In the solar cell of the present invention, the n-type transparentconductive film preferably is an ITO film, a SnO₂ film, an In₂O₃ film, aZnO:Al film, or a ZnO:B film. It should be noted that the n-typetransparent conductive film in the solar cell of the present inventionmay have the same configuration as that of a n-type transparentconductive film of any known solar cell.

The substrate preferably is a substrate that contains an element ofGroup Ia (alkali metal element). It should be noted that the “Group Ia”refers to “Group 1” according to the long-form periodic tablerecommended by IUPAC. In the case where an element of Group Ia iscontained in the substrate, in the formation of the p-type semiconductorcrystal of the compound semiconductor layer, the element of Group Ia ofthe substrate is diffused in the p-type semiconductor crystal via aconductive film, whereby the crystallinity of the p-type semiconductorcrystal is enhanced. Further, it is preferable that a difference betweena coefficient of linear expansion of the substrate and a coefficient oflinear expansion of the p-type semiconductor crystal is small. This isbecause in the case where the difference is small, crystal defects ofthe p-type semiconductor crystal are reduced. Accordingly, in the solarcell of the present invention, it is preferable that the substrate is aglass substrate containing at least one alkali metal element selectedfrom the group consisting of Na (sodium), K (potassium), and Li(lithium), and a difference between a coefficient of linear expansion ofthe substrate and a coefficient of linear expansion of the p-typesemiconductor crystal is within a range of not less than 1×10⁻⁶/K(Kelvin) and not more than 3×10⁻⁶/K.

The conductive film preferably is a metal film such as a Mo (molybdenum)film, a Cr (chromium) film, a Au (gold) film, or a Pt (platinum) film.It should be noted that the conductive film in the solar cell of thepresent invention may have the same configuration as that of aconductive film in any known solar cell.

The following describes the solar cell producing method of the presentinvention. The solar cell producing method of the present inventionincludes, as described above, the steps of forming a conductive film;growing a p-type semiconductor crystal; forming a n-type window layerhaving an aperture; doping a n-type impurity in the p-type semiconductorcrystal, in the vicinity of a surface of the p-type semiconductorcrystal under the aperture of the n-type window layer; and forming an-type transparent conductive film. By carrying out the step of dopingthe n-type impurity in the p-type semiconductor crystal after the n-typewindow layer having an aperture is formed, the high-resistance part canbe formed selectively in the vicinity of the surface of the p-typesemiconductor crystal under the aperture of the n-type window layer. Itshould be noted that the part of the p-type semiconductor crystal inwhich the n-type impurity is not doped becomes the low-resistance part.

In the growth of the p-type semiconductor crystal, in the case where thep-type semiconductor crystal is a chalcopyrite-structured Cu(Ga,In)Se₂crystal, the crystal preferably is grown at a film formation rate in arange of not less than 0.2 μm/min. and not more than 2 μm/min. This isbecause if the film formation rate. is less than 0.2 μm/min., the p-typesemiconductor crystal has smaller recesses and projections on its face,resulting in excellent coverage of the n-type window layer, whereas thetime for growing the p-type semiconductor crystal increases, resultingin deterioration of manufacturability. However, even in the case wherethe film formation rate is less than 0.2 μm/min., the yield can beimproved since the coverage defects accidentally occurring in the n-typewindow layer can be compensated. On the other hand, it is because if thefilm formation rate exceeds 2 μm/min., the crystallinity of the p-typesemiconductor crystal deteriorates, which makes it difficult to use thep-type semiconductor crystal as a light-absorption layer of a solarcell. The foregoing crystal more preferably is grown at a film formationrate in a range of not less than 0.5 μm/min. and not more than 1.5μm/min. In the case where the p-type semiconductor crystal is formed atthe film formation rate in the foregoing range, the manufacturabilitycan be improved significantly without a substantial decrease in theenergy conversion efficiency, as compared with the case where the p-typesemiconductor crystal is grown at a film formation rate of less than 0.2μm/min. as conventionally.

In the solar cell producing method of the present invention, in the stepof doping the n-type impurity in the p-type semiconductor crystal, thefollowing method can be applied: an impurity film is formed bydepositing the n-type impurity by a vapor deposition method or anevaporation method on the n-type window layer and the portion of thep-type semiconductor crystal that is exposed at the aperture of then-type window layer, and the n-type impurity in the impurity film isdiffused by a heat treatment into the portion of the p-typesemiconductor crystal (this method hereinafter also is referred to as“first method”). With the first method, it is possible to form ahigh-resistance part in the inside of the p-type semiconductor crystalsurely. Further, by controlling the film thickness of the impurity film,and the treating temperature and time in the heat treatment, ahigh-resistance part that has a desired size and contains the n-typeimpurity at a desired concentration can be formed easily.

In the first method, in the deposition of the n-type impurity, then-type impurity may be deposited while varying a direction of depositionof the impurity with respect to the substrate. For instance, thesubstrate is rotated while being inclined at a desired angle withrespect to the direction of deposition of the n-type impurity, or thesubstrate is moved so that a normal of a surface of the substratedefines a conical surface having the direction of deposition of then-type impurity as its central axis, or further alternatively, thesubstrate is moved, along with a movement such as those described above,so that an angle formed between the normal direction of the surface ofthe substrate and the direction of deposition of the n-type impurity isvaried. By applying this method, even if the exposed surface of thecompound semiconductor layer that is exposed at the aperture of then-type window layer is in a sinus-like complex recessed surface, then-type impurity can be deposited in a large area of the exposed surfaceexcellently.

In the solar cell producing method of the present invention, in the stepof doping the n-type impurity in the p-type semiconductor crystal, thefollowing method can be applied: an impurity film is formed bydepositing the n-type impurity by plating on the portion of the p-typesemiconductor crystal that is exposed at the aperture of the n-typewindow layer, and the n-type impurity in the impurity film is diffusedby a heat treatment into the portion of the p-type semiconductor crystal(this method hereinafter also is referred to as “second method”). Withthe second method, it is possible to form a high-resistance part in theinside of the p-type semiconductor crystal surely. Further, since it ispossible to form the impurity film selectively on the exposed portion ofthe p-type semiconductor crystal that is exposed at the aperture of then-type window layer, it is possible to form the high-resistance part atan effective location efficiently. Still further, even if the exposedsurface of the p-type semiconductor crystal that is exposed at theaperture of the n-type window layer is in a sinus-like complex recessedsurface, the impurity can be deposited in the sinus-like complexrecessed surface. Still further, by controlling the film thickness ofthe impurity film, and the treating temperature and time in the heattreatment, a high-resistance part that has a desired size and containsthe n-type impurity at a desired concentration can be formed easily.

The first and second methods of the present invention preferably furtherinclude the step of removing the impurity film, between the step ofdoping the n-type impurity in the p-type semiconductor crystal and thestep of forming the n-type transparent conductive film. If the impurityfilm remains, a resistance of a series resistor 2 (R_(s)) in anequivalent circuit of the solar cell shown in FIG. 1 increases, therebydeteriorating the properties of the solar cell such as an energyconversion efficiency, curve factors, etc.

In the solar cell producing method of the present invention, thefollowing method can be applied: in the step of doping the n-typeimpurity in the p-type semiconductor crystal, the n-type impurity isimplanted by ion implantation into the portion of the p-typesemiconductor crystal via the aperture of the n-type window layer (thismethod hereinafter also is referred to as “third method”). With thethird method, it is possible to form high-resistance part in the insideof the p-type semiconductor crystal surely. Further, by adjusting a doseamount, it is possible to form the high-resistance part containing then-type impurity at a desired concentration easily.

In the third method, in the ion implantation, the n-type impurity may beimplanted while varying a direction in which the n-type impurity ionsare implanted with respect to the substrate. For instance, the substrateis rotated while being inclined at a desired angle with respect to thedirection of the implantation of the n-type impurity ions, or thesubstrate is moved so that a normal of a surface of the substratedefines a conical surface having the direction of implantation of then-type impurity ions as its central axis, or further alternatively, thesubstrate is moved, along with a movement such as those described above,so that an angle formed between the normal direction of the surface ofthe substrate and the direction of implantation of the n-type impurityions is varied. By applying such a method, even if the exposed surfaceof the p-type semiconductor crystal that is exposed at the aperture ofthe n-type window layer is in a sinus-like complex recessed surface, then-type impurity can be implanted in a large area of the exposed surfaceexcellently.

In the third method, in the step of doping the n-type impurity in thep-type semiconductor crystal, it is preferable that a heat treatment iscarried out additionally, after the n-type impurity is implanted. Thisis because by carrying out the heat treatment, damage occurring upon theimplantation of the n-type impurity ions can be alleviated, and then-type impurity ions thus implanted can be diffused in the p-typesemiconductor crystal. Further, by controlling the treating temperatureand time in the heat treatment, a high-resistance part that has adesired size and contains the n-type impurity at a desired concentrationcan be formed easily.

The first, second, and third method of the present invention further mayinclude the step of forming a n-type buffer layer having an aperture,between the step of growing the p-type semiconductor crystal and thestep of forming the n-type window layer. With such a method, the solarcell that includes the n-type buffer layer formed between the p-typesemiconductor crystal and the n-type window layer can be produced.

To form the conductive film, the n-type window layer, the n-type bufferlayer, and the n-type transparent conductive film, any known techniquesmay be used.

EMBODIMENT 1

An embodiment of a solar cell of the present invention produced by theabove-mentioned first method is described as Embodiment 1, withreference to FIGS. 2 and 3A to 3D. FIG. 2 is a schematic cross-sectionalview illustrating a configuration of a solar cell according toEmbodiment 1. FIGS. 3A to 3D are schematic cross-sectional views showingrespective steps of the first method for producing the solar cellaccording to Embodiment 1. It should be noted that FIG. 3A shows a stepof stacking a conductive film, a p-type semiconductor crystal, and an-type window layer on a substrate, FIG. 3B shows a step of forming animpurity film by vapor deposition, FIG. 3C shows a step of diffusing an-type impurity, and FIG. 3D shows a step of removing the impurity film.

The solar cell shown in FIG. 2 includes a substrate 21, a conductivefilm 22 formed on the substrate 21, a compound semiconductor layer 23formed on the conductive film 22, a n-type window layer 24 with a n-typeconductivity that has pinholes 29 (apertures) formed on the compoundsemiconductor layer, and a n-type transparent conductive film 25 formedon the n-type window layer 24 and exposed portions of the compoundsemiconductor layer 23 that are exposed in the pinholes 29 in the n-typewindow layer 24. The compound semiconductor layer 23 includes alow-resistance part 23A having the p-type conductivity formed on theconductive film 22, and high-resistance parts 23B that are formed on thelow-resistance part 23A and under the pinholes 29 in the n-type windowlayer 24 and are doped with a n-type impurity. The n-type transparentconductive film 25 preferably is connected with the low-resistance part23A only via either the high-resistance parts 23B or the n-type windowlayer 24.

In the solar cell shown in FIG. 2, the substrate 21 preferably is asubstrate containing an element of Group Ia (alkali metal) such as Na.The conductive film 22 preferably is a metal film such as a Mo film. Thep-type semiconductor crystal of the compound semiconductor layer 23preferably is a chalcopyrite-structured Ib-IIIb-VIb crystal having thep-type conductivity, such as CuInSe₂ crystal, Cu(Ga,In)Se₂ crystal,CuIn(S,Se)₂ crystal, or the like. The impurity doped in thehigh-resistance parts 23B preferably is Zn. The n-type window layer 24preferably is a n-type ZnMgO film. The n-type transparent conductivefilm 25 preferably is any one of an ITO film, a SnO₂ film, an In₂O₃film, a ZnO:Al film, or a ZnO:B film. Further, a difference between acoefficient of linear expansion of the substrate 21 and a coefficient oflinear expansion of the p-type semiconductor crystal preferably iswithin a range of not less than 1×10⁻⁶/K and not more than 3×10⁻⁶/K.

The solar cell according to Embodiment 1 having the structure shown inFIG. 2 is produced in the following manner. First, as shown in FIG. 3A,the conductive film 22 is formed on the substrate 21 by sputtering. Itshould be noted that the conductive film 22 preferably has a sheetresistance of not more than 0.5 Ω/□. For instance, by sputtering, a Mofilm with a film thickness of approximately 0.4 μm is formed.

Next, as shown in FIG. 3A, the p-type semiconductor crystal 33 is formedon the conductive film 22 by multisource vapor deposition orselenidation. For instance, in the case where the p-type semiconductorcrystal 33 is chalcopyrite-structured Cu(Ga,In)Se₂ crystal, themultisource vapor deposition employing Cu, Ga, In, and Se as depositionsources can be used. Here, the film preferably is grown at a filmformation rate in a range of not less than 0.5 μm/min. and not more than1.5 μm/min. On the other hand, in the case where selenidation is used, aCuGa/In/Se precursor is formed by sputtering, and subsequently theCuGa/In/Se precursor is heated to approximately 450° C. to 550° C., sothat a Cu(Ga,In)Se₂ film is formed by solid phase diffusion.Alternatively, a CuGa/In precursor is formed by sputtering, andsubsequently the CuGa/In precursor is subjected to a heat treatment in aH₂Se gas, so that a Cu(Ga,In)Se₂ film is formed. The p-typesemiconductor crystal 33 formed by multisource vapor deposition orselenidation consequently has an uneven surface on its upper face.

Next, as shown in FIG. 3A, the n-type window layer 24 is formed on theuneven surface of the p-type semiconductor crystal 33 by sputtering or asolution method. For instance, a ZnMgO film with a film thickness ofapproximately 100 nm is formed as the n-type window layer 24 by asputtering method employing ZnO and MgO as targets. Since the p-typesemiconductor crystal 33 has an uneven surface on its upper face,pinholes 29 (apertures) are formed in the n-type window layer 24.

Next, as shown in FIG. 3B, Zn as a n-type impurity is deposited fromabove the n-type window layer 24 by vapor deposition or CVD (chemicalvapor deposition), so that an impurity film 36 is formed on the n-typewindow layer 24 and inside the pinholes 29 in the n-type window layer24. For instance, a Zn film with a film thickness of approximately 20 nmis formed as the impurity film 36.

Next, the obtained stack composed of the substrate 21, the conductivefilm 22, the p-type semiconductor crystal 33, the n-type window layer24, and the impurity film 36 is annealed (heat treatment). For instance,it is heated at 170° C. in a nitrogen atmosphere for 20 minutes. By sodoing, the n-type impurity is diffused to the inside of the p-typesemiconductor crystal 33 from portions of the impurity film 36 in directcontact with the p-type semiconductor crystal 33 (the impurity filmportions inside the pinholes 29), and as shown in FIG. 3C, the compoundsemiconductor layer 23 that includes the high-resistance parts 23Bcontaining the n-type impurity diffused from the impurity film 36 andthe low-resistance part 23A not containing the n-type impurity isformed.

Next, as shown in FIG. 3D, the impurity film 36 remaining on thecompound semiconductor layer 23 and the n-type window layer 24 isremoved by etching. For the removal of the impurity film 36, the dryetching technique may be used, but to simply and surely removesubstantially only the impurity film 36, the wet etching preferably isused in which the stack is brought into contact with an etching solutionsuch as hydrochloric acid. For instance, the stack is immersed inhydrochloric acid for several seconds. After the impurity film 36 (seeFIG. 3C) is removed, the stack composed of the substrate 21, theconductive film 22, the compound semiconductor layer 23 including thelow-resistance part 23A and the high-resistance parts 23B, and then-type window layer 24 is washed with a washing liquid such as purewater.

Next, as shown in FIG. 2, the n-type transparent conductive film 25 isformed by sputtering on the exposed surfaces of the compoundsemiconductor layer 23 exposed in the pinholes 29 of the n-type windowlayer 24, as well as on the n-type window layer 24. Through theforegoing steps, the solar cell according to Embodiment 1 having theconfiguration shown in FIG. 2 is produced.

The solar cell according to Embodiment 1 produced through the stepsshown in FIGS. 3A to 3D is characterized in that the resistance of theshunt resistor 1 (see FIG. 1) of the equivalent circuit of the solarcell can be increased surely, as compared with a solar cell of acomparative example produced without going through the steps shown inFIGS. 3B to 3D that accordingly does not contain the n-type impurity inthe high-resistance parts 23B. Further, the resistance of the shuntresistor 1 of the equivalent circuit of the solar cell according toEmbodiment 1 can be set to not less than 5 times the resistance of thecounterpart of the solar cell of a comparative example corresponding toEmbodiment 1, or can be set to not less than 2 kΩ·cm², which is apreferable value for a solar cell. Still further, the solar cellaccording to Embodiment 1 can be configured so as to have an energyconversion efficiency of not less than 17%, and to have a n value as thediode index of not more than 1.5.

The above description refers to a configuration that does not include abuffer layer, but a buffer layer may be formed between the compoundsemiconductor layer and the n-type window layer as required.

EMBODIMENT 2

An embodiment of a solar cell produced by the above-mentioned secondmethod is described as Embodiment 2, with reference to FIGS. 4A to 4D.FIGS. 4A to 4D are schematic cross-sectional views showing respectivesteps of the second method for producing the solar cell according toEmbodiment 2. It should be noted that FIG. 4A shows a step of forming animpurity film by plating, FIG. 4B shows a state after the completion ofthe step of forming the impurity film, FIG. 4C shows a step of diffusinga n-type impurity, and FIG. 4D shows a step of removing the impurityfilm.

The solar cell according to Embodiment 2 is produced by the same methodas the method for producing the solar cell according to Embodiment 1except for the manner of how the high-resistance parts are formed.Further, the structure of the solar cell according to Embodiment 2 issubstantially the same as the solar cell according to Embodiment 1 shownin FIG. 2. Therefore, only the second method for producing the solarcell according to Embodiment 2 is described here. It should be notedthat FIG. 2 is referred to as required.

The solar cell according to Embodiment 2 is produced in the followingmanner. First, as shown in FIG. 4A, the conductive film 22, the p-typesemiconductor crystal 33, and the n-type window layer 24 having thepinholes 29 are stacked on the substrate 21 in the stated order by thesame method as that for Embodiment 1 described above, whereby a stack isformed.

Next, as shown in FIG. 4A, after the stack is immersed in anelectroplating solution 42 contained in a solution vessel, a voltage isapplied by using an electrode 41 formed with Zn metal and provided inthe electroplating solution 42 as an anode and the conductive film 22 inthe stack as a cathode. The application of a voltage causes an ionizedn-type impurity (n-type impurity ions) to be dissolved into theelectroplating solution 42 from the electrode 41, and the dissolvedn-type impurity ions are deposited selectively on the exposed surfacesof the p-type semiconductor crystal 33 exposed in the pinholes 29 of then-type window layer 24.

Here, the selective deposition of the n-type impurity ions is described.If the n-type window layer 24 has a high resistance, the n-type windowlayer 24 and the compound semiconductor layer 23 forms a pn junction towhich a voltage is applied in a reverse bias state, which causessubstantially no electric current to run via the n-type window layer 24having the n-type conductivity. Consequently, none of, or at most, atrace of the n-type impurity ions are deposited on the n-type windowlayer 24. On the other hand, since electric current runs via the exposedsurfaces of the p-type semiconductor crystal 33 exposed in the pinholes29 of the n-type window layer 24, the n-type impurity ions are depositedon the foregoing surfaces. Thus, it is possible to cause the n-typeimpurity ions to be deposited selectively on the exposed surfaces of thep-type semiconductor crystal 33 exposed in the pinholes 29 of the n-typewindow layer 24. With this, as shown in FIG. 4B, impurity films 46 areformed selectively on the exposed portions of the p-type semiconductorcrystal 33 exposed in the pinholes 29 of the n-type window layer 24. Forinstance, Zn films with a film thickness of approximately 20 nm areformed as the impurity films 46.

Next, the stack in which the impurity film 46 is formed is annealed(heat treatment) so that the n-type impurity composing the impurity film46 is diffused to the inside of the p-type semiconductor crystal 33. Forinstance, it is heated at 170° C. in a nitrogen atmosphere for 20minutes. By so doing, the n-type impurity is diffused to the inside ofthe p-type semiconductor crystal 33 from the impurity films 46 in directcontact with the p-type semiconductor crystal 33 (the impurity filmsinside the pinholes 29), and as shown in FIG. 4C, the compoundsemiconductor layer 23 that includes the high-resistance parts 23Bcontaining the n-type impurity diffused from the impurity films 46 andthe low-resistance part 23A not containing the n-type impurity diffusedfrom the impurity films 46 is formed.

Next, as shown in FIG. 4D, it is preferable that the impurity films 46remaining on the compound semiconductor layer 23 are removed by the samemethod as that in Embodiment 1 described above. After the removal of theimpurity films 46 (see FIG. 4C), the stack composed of the substrate 21,the conductive film 22, the compound semiconductor layer 23 includingthe low-resistance part 23A and the high-resistance parts 23B, and then-type window layer 24 is washed.

Next, as shown in FIG. 2, the n-type transparent conductive film 25 isformed by sputtering or CVD on the exposed surfaces of the compoundsemiconductor layer 23 exposed in the pinholes 29 of the n-type windowlayer 24, as well as on the n-type window layer 24. Through theforegoing steps, the solar cell according to Embodiment 2 is produced.

The solar cell according to Embodiment 2 produced through the stepsshown in FIGS. 4A to 4D is characterized in that the resistance of theshunt resistor 1 (see FIG. 1) of the equivalent circuit of the solarcell can be increased surely, as compared with a solar cell of acomparative example produced without going through the steps shown inFIGS. 4B to 4D that accordingly does not contain the n-type impurity inhigh-resistance parts 23B. Further, the resistance of the shunt resistor1 of the equivalent circuit of the solar cell according to Embodiment 2can be set to not less than 5 times the resistance of the counterpart ofthe solar cell of a comparative example corresponding to Embodiment 2,or can be set to not less than 2 kΩ·cm², which is a preferable value fora solar cell. Still further, the solar cell according to Embodiment 2can be configured so as to have an energy conversion efficiency of notless than 17%, and to have a n value as the diode index of not more than1.5.

EMBODIMENT 3

An embodiment of a solar cell of the present invention produced by theabove-mentioned third method is described as Embodiment 3, withreference to FIGS. 5A to 5C. FIGS. 5A to 5C are schematiccross-sectional views showing respective steps of the third method forproducing the solar cells of the present invention. It should be notedthat FIG. 5A shows a step of stacking a conductive film, a p-typesemiconductor crystal, and a n-type window layer on a substrate, FIG. 5Bshows a step of implanting a n-type impurity by ion implantation, andFIG. 5C shows a step of forming high-resistance parts by diffusing then-type impurity thus implanted.

The solar cell according to Embodiment 3 is produced by the same methodas the methods for producing the solar cells according to Embodiments 1and 2 except for the manner of how high-resistance parts are formed.Further, the configuration of the solar cell according to Embodiment 3is substantially the same as the above-described solar cell according toEmbodiment 1 shown in FIG. 2. Therefore, only the third method forproducing the solar cell according to Embodiment 3 is described here. Itshould be noted that FIG. 2 is referred to as required.

The solar cell according to Embodiment 3 is produced in the followingmanner. First, as shown in FIG. 5A, the conductive film 22, the p-typesemiconductor crystal 33, and the n-type window layer 24 having thepinholes 29 are stacked on the substrate 21 in the stated order by thesame method as that for Embodiment 1 described above, whereby a stack isformed.

Next, as shown in FIG. 5B, an ionized n-type impurity (n-type impurityions) is implanted in the p-type semiconductor crystal 33 of the stack,from the side of the stack where the p-type semiconductor crystal 33 isformed. By so doing, the n-type impurity ions are implanted in thep-type semiconductor crystal 33 via the pinholes 29 of the n-type windowlayer 24, whereby ion-implanted parts 56B are formed inside the p-typesemiconductor crystal 33 under the pinholes 29 of the n-type windowlayer 24. As the n-type impurity, Zn, Mg, or Ca is preferred. It shouldbe noted that the part of the p-type semiconductor crystal 33 in whichthe impurity ions are not implanted is a non-ion-implanted part 56A. Forinstance, in the case where the n-type impurity ions are Zn ions, the Znions accelerated with an energy of approximately 50 keV are implanteduntil the dose amount becomes not less than 5×10¹⁵/cm² and not more than5×10¹⁶/cm², whereby the ion-implanted parts 56B are formed. In thiscase, the Zn ions intrude to a depth of approximately 0.01 μm to 0.05μm. It should be noted that the acceleration energy and the dose amountare to be adjusted appropriately according to the type of an elementused as an impurity ion, the type of the p-type semiconductor crystal 33subjected to the ion implantation, etc.

Next, after the ion-implanted parts 56B are formed, the stack includingthe substrate 21, the conductive film 22, the p-type semiconductorcrystal 33 having the non-ion-implanted part 56A and the ion-plantedparts 56B, and the n-type window layer 24 is annealed (heat treatment),so that the n-type impurity in the ion-implanted parts 56B are diffusedin the non-ion-implanted part 56A. By so doing, the compoundsemiconductor layer 23 that includes the high-resistance parts 23Bcontaining the n-type impurity and the low-resistance part 23A notcontaining the n-type impurity diffused from the ion-implanted parts 56Bis formed, as shown in FIG. 5C. Besides, this annealing allows thecompound semiconductor layer 23 (p-type semiconductor crystal 33) to berestored from damage that occurs upon the ion implantation.

Next, as shown in FIG. 2, the n-type transparent conductive film 25having the n-type conductivity is formed by sputtering on the exposedsurfaces of the compound semiconductor layer 23 exposed in the pinholes29 of the n-type window layer 24, as well as on the n-type window layer24. Through the foregoing steps, the solar cell according to Embodiment3 is produced.

The solar cell according to Embodiment 3 produced through the stepsshown in FIGS. 5A to 5C is characterized in that the resistance of theshunt resistor 1 (see FIG. 1) of the equivalent circuit of the solarcell can be increased surely, as compared with a solar cell of acomparative example produced without going through the steps shown inFIGS. 5B and 5C that accordingly does not contain the n-type impurity inthe high-resistance parts 23B. Further, the resistance of the shuntresistor of the equivalent circuit of the solar cell according toEmbodiment 3 can be set to not less than 5 times the resistance of thecounterpart of a solar cell of a comparative example corresponding toEmbodiment 3, or can be set to not less than 2 kΩ·cm², which is apreferable value for a solar cell. Still further, the solar cellaccording to Embodiment 3 can be configured so as to have an energyconversion efficiency of not less than 17%, and a n value as the diodeindex of not more than 1.5.

EMBODIMENT 4

An embodiment of a solar cell having a n-type buffer layer is describedas Embodiment 4, with reference to FIG. 6. It should be noted that thesolar cell according to Embodiment 4 has the same configuration as thatof the solar cells according to Embodiments 1 to 3 described aboveexcept that the n-type window layer is a ZnO film and the solar cellincludes a n-type buffer layer. Therefore, the same members aredesignated with the same referential numerals and the detaileddescriptions of the same are omitted.

The solar cell shown in FIG. 6 includes a substrate 21, a conductivefilm 22, a compound semiconductor layer 23, a n-type window layer 24that has the n-type conductivity and includes pinholes 29 (apertures), an-type transparent conductive film 25, and a n-type buffer layer 26 thatis formed between the compound semiconductor layer 23 and the n-typewindow layer 24 and that includes pinholes 39 connected with thepinholes 29 of the n-type window layer 24. It is preferable that then-type window layer 24 is a ZnO film, and the n-type buffer layer 26 isa Zn(O,OH) film or a Zn(O,S,OH) film.

The solar cell according to Embodiment 4 is produced in the followingmanner. In the method for producing the solar cell according toEmbodiment 4, the n-type buffer layer 26 is formed by a solution methodafter the p-type semiconductor crystal is grown and before the n-typewindow layer 24 is formed in the above-described first to thirdproducing methods. For instance, the n-type buffer layer 26 with a filmthickness of approximately 100 nm is formed. In the case where then-type buffer layer 26 is a Zn(O,OH) film, after the p-typesemiconductor crystal is grown, a stack including the substrate 21, theconductive film 22, and the p-type semiconductor crystal is brought intocontact with ammonia water whose pH is adjusted to approximately 7 to12, whose liquid temperature is kept at 50° C. to 80° C., and in which azinc salt is dissolved, whereby a Zn(O,OH) film can be formed. Further,in the case where the n-type buffer layer 26 is a Zn(O,S,OH) film, astack including the substrate 21, the conductive film 22, and the p-typesemiconductor crystal is brought into contact with ammonia water whosepH is adjusted to approximately 7 to 12, whose liquid temperature iskept at 50° C. to 80° C., and in which a zinc salt and asulfur-containing salt are dissolved, whereby a Zn(O,S, OH) film can beformed. Since the p-type semiconductor crystal has an uneven surface onits top face, the pinholes 39 (apertures) are formed in the n-typebuffer layer 26.

After the n-type buffer layer 26 is formed, a ZnO film as the n-typewindow layer 24 is formed on the uneven surface of the n-type bufferlayer 26 by sputtering or a solution method. For instance, a ZnO filmwith a film thickness of approximately 100 nm is formed by a sputteringmethod employing ZnO as a target. Since the p-type semiconductor crystaland the n-type buffer layer 26 have uneven surfaces on their top faces,the pinholes 29 (apertures) are formed in the n-type window layer. Theremaining steps are performed in the same manner as those of the firstto third methods described above, whereby the solar cell according toEmbodiment 4 is produced.

The solar cell according to Embodiment 4 is characterized in that theresistance of the shunt resistor 1 (see FIG. 1) of the equivalentcircuit of the solar cell can be increased surely, as compared with asolar cell of a comparative example having the same configuration exceptthat the solar cell does not contain the n-type impurity in thehigh-resistance parts 23B. Further, the resistance of the shunt resistorof the equivalent circuit of the solar cell according to Embodiment 4can be set to not less than 5 times the resistance of the counterpart ofthe solar cell of a comparative example corresponding to Embodiment 4,or can be set to not less than 2 kΩ·cm², which is a preferable value fora solar cell. Still further, the solar cell according to Embodiment 4can be configured so as to have an energy conversion efficiency of notless than 17%, and to have a n value as the diode index of not more than1.5.

EXAMPLE 1

An example of the above-described solar cell according to Embodiment 1is described below as Example 1. Here, FIGS. 2 and 3A to 3D are referredto. The solar cell of Example 1 shown in FIG. 2 was configured so thatthe substrate 21 was a soda lime glass substrate, the conductive film 22was a Mo film, the p-type semiconductor crystal of the compoundsemiconductor layer 23B was a chalcopyrite-structured p-typeCu(Ga,In)Se₂ crystal, the n-type impurity doped in the high-resistanceparts 23B was Zn, the n-type window layer 24 was a n-type ZnMgO film,and the n-type transparent conductive film 25 was an ITO film. Theresistance of the high-resistance parts 23 was set to be higher than theresistance of the n-type ZnMgO film. Further, the ITO film was formed soas to be connected with the low-resistance part 23A via substantiallyonly at least one of the n-type ZnMgO film and the high-resistance parts23B.

The solar cell of Example 1 was produced in the following manner. First,as shown in FIG. 3A, the Mo film with a film thickness of approximately400 nm as the conductive film 22 was formed on the soda lime glasssubstrate as the substrate 21 by sputtering or the like. The Mo film hada sheet resistance of approximately 0.5 Ω/□. Next, as shown in FIG. 3A,the chalcopyrite-structured p-type Cu(Ga,In)Se₂ crystal as the p-typesemiconductor crystal 33 was grown on the Mo film by vapor deposition,at a film formation rate of approximately 1 μm/min. until the film had athickness of approximately 2 μm in average. The p-type Cu(Ga,In)Se₂crystal had an uneven surface. Next, as shown in FIG. 3A, the n-typeZnMgO film as the n-type window layer 24 was formed on the p-typeCu(Ga,In)Se₂ crystal so as to have a film thickness of approximately 100nm by a sputtering method employing ZnO and MgO as targets. Since thep-type Cu(Ga,In)Se₂ crystal had an uneven surface, the pinholes 29(apertures) were formed in the n-type ZnMgO film. The pinholes 29 wereformed substantially in the vicinities of recessed portions of theuneven surface of the p-type Cu(Ga,In)Se₂ crystal.

Next, as shown in FIG. 3B, Zn was deposited by vapor deposition on then-type ZnMgO film and the exposed portions of the p-type Cu(Ga,In)Se₂crystal exposed in the pinholes 29 in the n-type ZnMgO film, so that aZn film with a film thickness of approximately 20 nm was formed as theimpurity film 36. Next, as shown in FIG. 3C, the stack composed of thesoda lime glass substrate, the Mo film, the p-type Cu(Ga, In)Se₂crystal, and the Zn film was annealed at 170 K for 20 minutes. Thiscaused Zn to be diffused in the inside of the p-type Cu(Ga,In)Se₂)crystal from portions of the Zn film in direct contact with the p-typeCu(Ga,In)Se₂ crystal (portions of the Zn film in the pinholes 29),whereby the compound semiconductor layer 23 including thehigh-resistance parts 23B containing Zn and the low-resistance part 23Anot containing Zn diffused from the Zn film was formed. Next, the stackin which the high-resistance parts 23B were formed was immersed in ahydrochloric acid solution prepared as an etching solution for threeminutes, so that the Zn film remaining on the p-type Cu(Ga,In)Se₂crystal and the n-type ZnMgO film was removed. Then, the stack fromwhich the Zn film was removed was raised from the hydrochloric acidsolution, and subsequently, surfaces thereof were washed with purewater.

Next, as shown in FIG. 2, an ITO film as the n-type transparentconductive film 25 was formed by sputtering on the n-type ZnMgO film andthe high-resistance parts 23B. With this, the production of the solarcell of Example 1 was completed.

The solar cell of Example 1 had a shunt resistance of approximately 3kΩ·cm², which was approximately 6 times the resistance of a solar cellaccording to Comparative Example 1 produced by the same method as thatfor the solar cell of Example 1 except that the step of forming thehigh-resistance parts 23B (impurity film) was not carried out,Comparative Example 1 accordingly being characterized in that Zn was notcontained in the high-resistance parts 23B. Further, the solar cell ofExample 1 had an energy conversion efficiency of 17.6%. Still further,though the p-type Cu(Ga,In)Se₂ crystal used in the solar cell of Example1 was a polycrystal, the solar cell of Example 1 had a n value (diodeindex) of 1.47, which is equal to that in the case where the crystalused therein is a single crystal.

EXAMPLE 2

An example of the above-described solar cell according to Embodiment 2is described below as Example 2, with reference to FIGS. 2 and 4A to 4D.The solar cell of Example 2 shown in FIG. 2 was configured so that thesubstrate 21 was a soda lime glass substrate, the conductive film 22 wasa Mo film, the p-type semiconductor crystal of the compoundsemiconductor layer 23 was a chalcopyrite-structured p-type Cu(Ga,In)Se₂crystal, the n-type impurity doped in the high-resistance parts 23B wasZn, the n-type window layer 24 was a n-type ZnMgO film, and the n-typetransparent conductive film 25 was an ITO film. The resistance of thehigh-resistance parts 23 was set to be higher than the resistance of then-type ZnMgO film. Further, the ITO film was formed so as to beconnected with the low-resistance part 23A via substantially only atleast one of the n-type ZnMgO film and the high-resistance parts 23B.

The solar cell of Example 2 was produced in the following manner. First,as shown in FIG. 4A, the Mo film with a film thickness of approximately400 nm was formed on the soda lime glass substrate by sputtering or thelike. The Mo film had a sheet resistance of approximately 0.5 Ω/□. Next,as shown in FIG. 4A, the chalcopyrite-structured p-type Cu(Ga,In)Se₂crystal as the p-type semiconductor crystal 33 was grown on the Mo filmby vapor deposition, at a film formation rate of 1 μm/min. until thefilm had a thickness of approximately 2 μm. The p-type Cu(Ga,In)Se₂crystal had an uneven surface. Next, as shown in FIG. 4A, the n-typeZnMgO film as the n-type window layer 24 was formed on the p-typesemiconductor crystal 33 so as to have a film thickness of approximately100 nm by a sputtering method employing ZnO and MgO as targets. Sincethe p-type Cu(Ga,In)Se₂ crystal had an uneven surface, the pinholes 29(apertures) were formed in the n-type ZnMgO film. The pinholes 29 wereformed substantially in the vicinities of recessed portions of theuneven surface of the p-type Cu(Ga,In)Se₂ crystal.

Next, as shown in FIG. 4A, the stack composed of the soda lime glasssubstrate, the Mo film, the p-type Cu(Ga, In)Se₂ crystal, and the n-typeZnMgO film is immersed in an electroplating solution 42 containing zincsulfate, which was contained in a solution vessel. After the stack wasimmersed, a voltage in a range of 0.5 V to 0.6 V was applied for threeminutes by using a Zn electrode 41 formed with Zn (n-type impurity) andprovided in the electroplating solution 42 as an anode and the Mo filmin the stack as a cathode. The application of a voltage caused Zn⁺ ions(n-type impurity ions) to be leached out of the Zn electrode 41 into theelectroplating solution 42, and the leached Zn⁺ ions were depositedselectively on the exposed surfaces of the p-type Cu(Ga,In)Se₂ crystalexposed in the pinholes 29 of the n-type ZnMgO film. By so doing, Znfilms with a film thickness of approximately 20 nm were formed as theimpurity films 46, as shown in FIG. 4B.

Next, the stack with the Zn films being formed therein was annealed at asubstrate temperature of 170° C. for 20 minutes. This caused Zn in theZn films to be diffused in the inside of the p-type Cu(Ga,In)Se₂)crystal, whereby the compound semiconductor layer 23 including thehigh-resistance parts 23B containing Zn and the low-resistance part 23Anot containing Zn diffused from the Zn films was formed as shown in FIG.4C. Next, by applying wet etching, the stack including the compoundsemiconductor layer 23 having the low-resistance part 23A and thehigh-resistance parts 23B was immersed in a hydrochloric acid solution,so that the Zn films remaining on the n-type ZnMgO film and thehigh-resistance parts 23B were removed. Then, the stack from which theZn films were removed was washed with pure water.

Next, as shown in FIG. 2, an ITO film as the n-type transparentconductive film 25 was formed by sputtering on the high-resistance parts23B and the n-type ZnMgO film. With this, the production of the solarcell of Example 2 was completed.

A shunt resistor 1 (see FIG. 1) of an equivalent circuit of the solarcell of Example 2 had a resistance of approximately 2 kΩ·cm², which wasapproximately 5 times the resistance of a solar cell according toComparative Example 2 produced by the same method as that for the solarcell of Example 2 except that the step of forming the high-resistanceparts 23B was not carried out, Comparative Example 2 accordingly beingcharacterized in that Zn was not contained in the high-resistance parts23B. Further, the solar cell of Example 2 had an energy conversionefficiency of 17%. Still further, though the p-type Cu(Ga,In)Se₂ crystalused in the solar cell of Example 2 was a polycrystal, the solar cell ofExample 2 had a n value (diode index) of 1.5, which is equal to that inthe case where the crystal used therein is a single crystal.

EXAMPLE 3

An example of the above-described solar cell according to Embodiment 3is described below as Example 3. It should be noted that FIGS. 2 and 5Ato 5C are referred to. The solar cell of Example 3 shown in FIG. 2 wasconfigured so that the substrate 21 was a soda lime glass substrate, theconductive film 22 was a Mo film, the p-type semiconductor crystal ofthe compound semiconductor layer 23 was a chalcopyrite-structured p-typeCu(Ga,In)Se₂ crystal, the n-type impurity doped in the high-resistanceparts 23B was Zn, the n-type window layer 24 was a n-type ZnMgO film,and the n-type transparent conductive film 25 was an ITO film. Theresistance of the high-resistance parts 23 was set to be higher than theresistance of the n-type ZnMgO film. Further, the ITO film was formed soas to be connected with the low-resistance part 23A via substantiallyonly at least one of the n-type ZnMgO film and the high-resistance parts23B.

The solar cell of Example 3 was produced in the following manner. First,as shown in FIG. 5A, the Mo film with a film thickness of approximately400 nm as the conductive film 22 was formed on the soda lime glasssubstrate as the substrate 21 by sputtering or the like. The Mo film hada sheet resistance of approximately 0.5 Ω/□. Next, as shown in FIG. 5A,the chalcopyrite-structured p-type Cu(Ga,In)Se₂ crystal as the p-typesemiconductor crystal 33 was grown on the Mo film by vapor deposition,at a film formation rate of 1 μm/min. until the film had a thickness ofapproximately 2 μm in average. The p-type Cu(Ga,In)Se₂ crystal had anuneven surface. Next, as shown in FIG. 5A, the n-type ZnMgO film as then-type window layer 24 was formed on the p-type Cu(Ga,In)Se₂ crystal soas to have a film thickness of approximately 100 nm by sputtering. Sincethe p-type Cu(Ga,In)Se₂ crystal had an uneven surface, the pinholes 29(apertures) were formed in the n-type ZnMgO film. The pinholes 29 wereformed substantially in the vicinities of recessed portions of theuneven surface of the p-type Cu(Ga,In)Se₂ crystal.

Next, as shown in FIG. 5B, Zn⁺ ions (n-type impurity ions) wereimplanted in the p-type Cu(Ga,In)Se₂ crystal via the pinholes 29 in then-type ZnMgO film, from the side where the n-type ZnMgO film was formed,until the dose amount became in a range of 5×10¹⁵/cm² to 5×10¹⁶/cm².With this, the ion-implanted parts 56B containing Zn were formed in thep-type Cu(Ga,In)Se₂ crystal.

Next, the stack provided with the p-type Cu(Ga,In)Se₂ crystal includingthe ion-implanted parts 56B and the non-ion-implanted part 56A in whichthe impurity ions were not implanted was annealed (heat treatment) at asubstrate temperature of 170° C. for 20 minutes, so that Zn thusimplanted therein was diffused in the p-type Cu(Ga,In)Se₂ crystal. By sodoing, the compound semiconductor layer 23 that included thehigh-resistance parts 23B containing Zn and the low-resistance part 23Anot containing Zn diffused from the Zn film was formed. This annealingalso allowed the p-type Cu(Ga,In)Se₂ crystal to be restored from damagethat occurred upon the ion implantation.

Next, as shown in FIG. 2, the n-type transparent conductive film 25 wasformed by sputtering on the n-type ZnMgO film and on surfaces of thehigh-resistance parts 23B. With this, the production of the solar cellof Example 3 was completed.

The shunt resistor 1 (see FIG. 1) of an equivalent circuit of the solarcell of Example 3 had a resistance of approximately 2 kΩ·cm², which wasapproximately 5 times the resistance of a solar cell according to acomparative example produced by the same method as that for the solarcell of Example 3 except that the step of implanting Zn ions and theannealing step were not carried out, the comparative example accordinglybeing characterized in that Zn was not contained in the high-resistanceparts 23B. Further, the solar cell of Example 3 had an energy conversionefficiency of 17%. Still further, though the p-type Cu(Ga,In)Se2 crystalused in the solar cell of Example 3 was a polycrystal, the solar cell ofExample 3 had a n value (diode index) of 1.5, which is equal to that inthe case where the crystal used therein is a single crystal.

INDUSTRIAL APPLICABILITY

The present invention is applicable for improving an energy conversionefficiency of a solar cell by increasing a resistance of a shuntresistor of an equivalent circuit of the solar cell. Further, thepresent invention is applicable for improving the manufacturability ofthe solar cell.

1. A solar cell comprising: a substrate; a conductive film formed on thesubstrate; a compound semiconductor layer formed on the conductive film,the compound semiconductor layer including a p-type semiconductorcrystal containing an element of Group Ib, an element of Group IIIb, andan element of Group VIb; a n-type window layer formed on the compoundsemiconductor layer, the n-type window layer having an aperture; and an-type transparent conductive film formed on the n-type window layer andon a portion of the compound semiconductor layer at the aperture of then-type window layer, wherein the compound semiconductor layer includes ahigh-resistance part, the high-resistance part being located in aportion of the compound semiconductor layer in the vicinity of a surfacethereof on a side opposite to the conductive film, the high-resistancepart containing a n-type impurity doped in the p-type semiconductorcrystal, and the high-resistance part is located under the aperture ofthe n-type window layer.
 2. The solar cell according to claim 1, whereinthe high-resistance part has a resistance higher than a resistance ofthe n-type window layer.
 3. The solar cell according to claim 1, whereinthe compound semiconductor layer has a recessed surface on its face onthe side opposite to the conductive film, and the high-resistance partis formed in the vicinity of the recessed surface.
 4. The solar cellaccording to claim 1, wherein the n-type transparent conductive film isconnected with a part of the compound semiconductor layer other than thehigh-resistance part only via at least either one of the n-type windowlayer and the high-resistance part.
 5. The solar cell according to claim1, wherein the high-resistance part contains, as the n-type impurity, atleast one element selected from the group consisting of the elements ofGroup IIa and the elements of Group IIb.
 6. The solar cell according toclaim 1, wherein the n-type impurity of the high-resistance part is Zn,Mg, or Ca.
 7. The solar cell according claim 1, wherein the p-typesemiconductor crystal of the compound semiconductor layer is achalcopyrite-structured CuInSe₂ crystal, a chalcopyrite-structuredCu(Ga,In)Se₂ crystal, or a chalcopyrite-structured CuIn(S,Se)₂ crystal.8. The solar cell according to claim 1, wherein the n-type window layeris a ZnO film or a ZnMgO film.
 9. The solar cell according to claim 1,further comprising: a n-type buffer layer formed between the compoundsemiconductor layer and the n-type window layer, the n-type buffer layerhaving an aperture that is connected with the aperture of the n-typewindow layer.
 10. The solar cell according to claim 9, wherein then-type buffer layer is a Zn(O,OH) film or a Zn(O,S,OH) film.
 11. Thesolar cell according to claim 1, wherein the n-type transparentconductive film is an ITO film, a SnO₂ film, an In₂O₃ film, a ZnO:Alfilm, or a ZnO:B film.
 12. The solar cell according to claim 1, whereinthe substrate is a glass substrate containing at least one alkali metalelement selected from the group consisting of Na, K, and Li, and adifference between a coefficient of linear expansion of the substrateand a coefficient of linear expansion of the p-type semiconductorcrystal is within a range of not less than 1×10⁻⁶/K and not more than3×10⁻⁶/K.
 13. A solar cell producing method comprising the steps of:forming a conductive film on a substrate; growing a p-type semiconductorcrystal on the conductive film, the p-type semiconductor crystalcontaining an element of Group Ib, an element of Group IIIb, and anelement of Group VIb; forming a n-type window layer on the p-typesemiconductor crystal, the n-type window layer having an aperture; andforming a n-type transparent conductive film on the n-type window layerand on a portion of the p-type semiconductor crystal at the aperture ofthe n-type window layer, the solar cell producing method furthercomprising the step of doping an n-type impurity in the p-typesemiconductor crystal, in the vicinity of a surface of the p-typesemiconductor crystal under the aperture of the n-type window layer, thedoping step being carried out between the step of forming the n-typewindow layer and the step of forming the n-type transparent conductivefilm.
 14. The solar cell producing method according to claim 13, whereinin the step of doping the n-type impurity in the p-type semiconductorcrystal, an impurity film is formed by depositing the n-type impurity bya vapor deposition method or an evaporation method on the n-type windowlayer and the portion of the p-type semiconductor crystal that isexposed at the aperture of the n-type window layer, and the n-typeimpurity in the impurity film is diffused by a heat treatment into theportion of the p-type semiconductor crystal.
 15. The solar cellproducing method according to claim 13, wherein in the step of dopingthe n-type impurity in the p-type semiconductor crystal, an impurityfilm is formed by depositing the n-type impurity by plating on theportion of the p-type semiconductor crystal that is exposed at theaperture of the n-type window layer, and the n-type impurity in theimpurity film is diffused by a heat treatment into the portion of thep-type semiconductor crystal.
 16. The solar cell producing methodaccording to claim 14, further comprising the step of removing theimpurity film, the step of removing the impurity film being carried outbetween the step of doping the n-type impurity in the p-typesemiconductor crystal and the step of forming the n-type transparentconductive film.
 17. The solar cell producing method according to claim15, further comprising the step of removing the impurity film, the stepof removing the impurity film being carried out between the step ofdoping the n-type impurity in the p-type semiconductor crystal and thestep of forming the n-type transparent conductive film.
 18. The solarcell producing method according to claim 13, wherein in the step ofdoping the n-type impurity in the p-type semiconductor crystal, then-type impurity is implanted by ion implantation into the portion of thep-type semiconductor crystal via the aperture of the n-type windowlayer.
 19. The solar cell producing method according to claim 18,wherein in the step of doping the n-type impurity in the p-typesemiconductor crystal, a heat treatment is carried out additionally,after the n-type impurity is implanted.
 20. The solar cell producingmethod according to claim 13, further comprising the step of forming an-type buffer layer having an aperture, the step of forming the n-typebuffer layer being carried out between the step of growing the p-typesemiconductor crystal and the step of forming the n-type window layer.